This doubles as a bug report, I think, but after trying to get whatever they give you for killing them both for a few hours I'm just really curious as to what I'm going through all this trouble to get. So firstly, I was wondering about that.

Secondly, I did manage to kill both of them just now with no reward. I used Weytwut to put Yin and Yang right by each other, killed one with fire, absorbed it's blood pool with blood mage and stood in its spot while I killed the other. Got a "One alive: false" message... and then nothing happened. I noticed a gold necklace at the bottom of the screen after I finished exploring but that seems like a pretty lackluster reward for all the trouble it takes to accomplish this.

On one final random side note, haven't found Dracul yet. Where does he like to hide?

I fought those two several times, and it never happens the same way somewhy.First time using a pretty powerful fighter i waltzed to one, deaded him, went to the second and croaked him too, both stayed dead, and i discovered later the loot at the bottom of the screen. Nothing more.

Second time, using a monk with the huge sword, I easily dispatched one, but needed to uncover some tiles before fighting the next, and the first one revived after the second died. Gold did appear at the bottom of the area thought. I kept killing one after the other, and for every kill the other respawned, and i got some gold at the bottom. Went through something like 350ish gold like that. Which somehow feels quite like a glitch.

As for Dracul, I got him from a subdungeon in Eastern Tundra, but it seems he can show up in different other places. Just keep on resetting the dungeon every time it's not him and you'll get there eventually.